Answer
An idle session can still be holding a lock on a table and its indexes (for example, after a network glitch or a user disconnecting without committing or rolling back a transaction), which blocks DDL-style operations like index validation from completing — identify the specific session holding a lock on the affected table (a common one being the Excel-parameter-cache table) and, after checking the business impact of doing so, terminate that long-running session before retrying the validation. As a general preventive measure, restarting the database after a delivery installation reduces how often this kind of lingering-session lock occurs.